This invention relates generally to high-energy lasers, and more particularly, to a novel solution to a problem posed by the need for a ground-based or space-based high-energy laser source of large effective aperture and power. The physics of monolithic high-energy lasers, such as excimer lasers, impose inherent limitations that preclude their operation at apertures greater than some level limited by considerations such as pump dimensions, the presence of parasites, optical element sizes, media uniformity, and so forth. For excimer lasers, energy levels greater than a fraction of one megajoule (MJ) in a pulsed mode of operation are difficult to achieve.
For lasers to be effectively used as defensive weapons, much higher energy levels are needed, and recent design efforts in this field have, therefore, focused on systems that employ arrays of lasers producing a single composite beam of very high energy. If an array of N beams, each of the same energy level, is appropriately focused onto a target, the energy intensity at the target will be in the order of N times the intensity resulting from just one of the beams. This assumes that the energy of the beams adds incoherently, i.e. that the separate beams are not in phase with each other. However, if the beams can be combined coherently, i.e. practically perfectly matched in frequency and phase, the energy intensity at the target will be approximately N.sub.2 times the intensity of a single beam. For an array of one hundred lasers, for example, there is a potential for increasing the target intensity by a factor of one hundred if the separate beams can be combined coherently rather than incoherently.
The concept of combining separate radiation beams coherently in phased arrays is well known in radio communications, but has been more difficult to put into practice for optical radiation. The difficulty, of course, stems from the difference in wavelengths between radio and optical waves. Even for radio transmissions at 1 GHz (gigahertz) and above, the wavelengths are measured in terms of centimeters or millimeters, and the construction of a phased array having mechanical tolerances of one twentieth of a wavelength are attainable without great difficulty. For optical radiation, however, the tolerances are very stringent. Light of wavelength 248 nm (nanometers), for example, requires tolerances of around 1.2.times.10.sup.-6 cm to achieve phase coherence to within one twentieth of a wavelength. Separate laser beams emanating from separate laser amplifiers are subject to separate sets of phase-aberrating conditions in the amplifiers and in the associated optical elements for each beam path. The resulting differences in phase arise not only from differences in construction and geometrical relationships, but also from factors that may vary with time. For example, optical components may be subject to mechanical "jitter" that causes phase and pointing changes, and the laser gain region within each amplifier may also change significantly with time.
Prior to this invention, optical phased-array technology has utilized principles of adaptive optics to achieve some degree of phase coherence. Basically, this approach employs one or more deformable mirrors, which are large reflecting surfaces made up of separately movable elements, each driven by a transducer, such as a piezoelectric device. The character of the optical wavefront emanating from such a mirror has to be sensed with a complex and highly sensitive interferometer, and then the composite wavefront has to be converted to electrical form, stored in an electronic memory, and manipulated mathematically to determine the magnitude of elemental corrections that have to be made in the deformable mirror.
The adaptive optics approach is inherently slow, because of its reliance on mechanical elements to effect phase compensation. The approach is also subject to errors due to intermirror optical path length differences, called "piston errors." Compensation of these errors has required the use of very complex arrangements of interferometry and adaptive optical components. The approach becomes even less practical as the size of the desired beam aperture increases. For large apertures, in the order of ten meters in diameter, deformable mirrors having as many as 10,000 elements may be required. Since each element is of finite size, the array has limited resolution and ability to correct wavefront distortions. Moreover, the cost and reliability of deformable mirrors of this magnitude have posed serious limitations to the development of a practical phased array system using adaptive optics.
By way of further background, the invention also relates to the field of phase conjugate optics. It has been recognized for some time that phase conjugation of light waves can be used to remove phase aberrations caused by the passage of a light beam through a distorting or phase-aberrating medium.
There is extensive literature on the subject of phase conjugate optics and the use of phase conjunction for the compensation of phase aberrations. A summary of the history and principles of phase conjugate optics is provided in a paper entitled "Phase Conjugate Optics and Real-Time Holography," by Amnon Yariv, IEEE Journal of Quantum Electronics, Vol. QE-14, No. 9, September, 1978, pp. 650-60.
Simply stated, a phase conjugation cell functions as a reflector with a special and useful property. When an incident light wave is focused into the cell, the reflected wave that emerges is the complex conjugate of the incident wave. The practical consequence of the phase conjugation is that the retro-reflected wave is as if it were "time-reversed" with respect to the incident wave. For example, if an incident wave, after passing through a distorting medium, has a bulge in its wavefront, representing a phase-lagging condition at a particular region of the front, this will be reflected as an opposite bulge, i.e. a phase lead, in the same region of the reflected wavefront. If the reflected wavefront then traverses the same distorting medium that caused the original bulge in the incident wavefront, the reflected wave will emerge from the distorting medium as an undistorted wave.
In spite of the existence of a large body of theoretical knowledge concerning the principles of phase conjugate optics, prior to the present invention these principles have not been applied to the problem with which the invention is concerned. It will be appreciated from the foregoing that there is still a critical need for an alternative approach to the construction of phased arrays of high-energy lasers. What is needed is a technique for coupling the outputs of multiple laser amplifiers together in frequency and phase, while at the same time eliminating "piston errors" between adjacent beams, and compensating for other sources of phase aberration in each beam path, at high resolution. The present invention provides a reliable and elegant solution to all of these problems, as will become apparent from the following summary of the inventive features.